Propylene oxidation in the presence of iridium metal

ABSTRACT

PROPYLENE IS OXIDIZED WITH OXYGEN TO ACETIC ACID, ACROLEIN, ETC. BY OXIDATION IN THE CONTACT PRESENCE OF IRIDIUM METAL.

United States Patent Oflice 3,644,510 Patented Feb. 22, 1972 3,644,510PROPYLENE OXIDATION IN THE PRESENCE OF IRIDIUM METAL Noel W. Cant andWilliam K. Hall, Pittsburgh, Pa., as-

signors to Gulf Research & Development Company, Pittsburgh, Pa. NoDrawing. Filed Nov. 25, 1968, Ser. No. 778,805 Int. Cl. C07c 53/08 US.Cl. 260-533 R 8 Claims ABSTRACT OF THE DISCLOSURE Propylene is oxidizedwith oxygen to acetic acid, acrolein, etc. by oxidation in the contactpresence of iridium metal.

This invention relates to a process for oxidizing propylene to obtain auseful mixture of organic compounds comprising acetic acid, acrolein andacetone.

In addition to the above desired compounds, by-products such as carbondioxide and water are also formed. It was expected that the partialoxidation of propylene would yield primarily organic acids containingthree carbon atoms, i.e. propionic acid. Quite unexpectedly, it has beenfound that the main product of the oxidation reaction is acetic acid solong as the oxidation reaction is carried out in the presence of iridiummetal.

In accordance with the invention, therefore, propylene is oxidized to aproduct comprising acetic acid by reacting propylene with a gascontaining free molecular oxygen in the contact presence of iridiummetal.

In order to obtain good selectivity to the desired oxygenated compoundsit is important that the propylene together with a gas containing freemolecular oxygen be contacted with the iridium metal either supported orunsupported at a temperature within the range of about 50 C. to about300 C., and preferably within the range of about 80 C. to about 200 C.Since the reaction is exothermic, means must be provided to control thetemperature of the reaction within the limits defined above. Below thelower temperature limits defined above the reaction rate becomes too lowto be economically feasible, whereas at temperatures above the upperlimits, the yield of desired oxygenated compounds decreases with theconcurrent production of excessive amounts of water and carbon dioxide.The temperature can be controlled by any suitable means, and one methodof at least partially controlling the temperature is to dilute theiridium metal by distending it on a suitable support material such assilica, magnesia, alumina, thoria or mixtures thereof. The iridium metalcan, of course, be used unsupported and when this is done the metal canbe in any suitable form, such as sponge form. When the iridium isdistended on a support, the surface area of the support is not criticaland can suitably be between 0.1 and 600 square meters per gram. Inaddition to the above named supports, materials such as carbon,kiesulguhr, pumice, the natural clays, mullite and alundum are alsosuitable. When the iridium metal is supported, suitable amounts ofiridium metal are from 0.2 to 30 weight percent of the total catalystwith preferred amounts from one of ten weight percent and the morepreferred amounts from one to five weight percent of the total catalyst.

The method of preparing the supported or unsupported catalysts is notcritical. Suitable methods of preparing the supported catalysts, forexample, include the method of incipient wetness using aqueous solutionsof suitable iridium salts such as H IrCl followed by drying andreduction in hydrogen to obtain the metal.

Another method of controlling the temperature is to dilute thepropylene-oxygen mixture with an inert gas such as nitrogen or helium.Yet another method of controlling the reaction temperature is to passthe admixture of propylene and oxygen over the iridium metal catalyst atvery high space velocities. Suitable gaseous space velocities are withinthe range of about one to about 2000 volumes of propylene measured atstandard temperature and pressure per volume of catalyst per hour, andthe preferred space velocities are from about five to about 200.

A total operating pressure of about one atmosphere is the desiredoperating pressure. Higher or lower pressures can be used; for example,pressures of from 0.5 to 15 atmospheres or more can suitably beemployed.

The ratio of the partial pressure of oxygen to the partial pressure ofpropylene can suitably be between 0.2 and 100 and is preferably betweentwo and 30. The partial pressure of propylene should be at least 0.05p.s.i.a. and is preferably from 0.1 to one p.s.i.a. when the totalpressure is atmospheric (14 p.s.i.a.). Correspondingly higher partialpressures of propylene would be employed at correspondingly higher totaloperating pressures.

The propylene is oxidized in the presence of a gas containing freemolecular oxygen. Pure oxygen can be used, but this creates problems oftemperature control as noted above. It is preferred that the freemolecular oxygen be diluted with an inert gas such as nitrogen orhelium. The volume percent of free molecular oxygen in the gascontaining it can suitably be between one and 100 and is preferablybetween one and 20. When propylene is mixed with this gas containingfree molecular oxygen, the partial pressure of oxygen is usually between0.5 and ten p.s.i.a. and is preferably between two and five p.s.i.a. Ifthe partial pressure of oxygen is below about 2.0 pounds per square inchabsolute, selectivity to the desired oxygenated products decreases,whereas above about two pounds per square inch absolute selectivity tothe desired oxygenated compounds remains substantially constant.

The invention will be further described with reference to the followingexperimental work. In all of the examples to follow the followingprocedure was employed. A single pass flow system was used wherein afeed mixture of propylene, oxygen and helium was passed through a bed(approximately two parts by volume of catalyst) of a supported iridiumcatalyst at a given temperature between 50 C. and 200 C. at a flow rateof between 2400 and 3000 volumes of total feed per hour. The contacttime was about 2.3 seconds and the space velocity based on the totalfeed was about 1500 volumes of feed per volume of catalyst per hour. Thepropylene space velocities for any particular run can be calculated bymultiplying 1500 by the ratio of the propylene partial pressure inmillimeters to the total pressure of about 740 millimeters. The reactionproducts were cooled to C. by indirect cooling to condense the reactionproduct which was mostly acetic acid with minor amounts of acetone,acrolein, acrylic and/or propionic acids. Water and CO were formed asby-products.

EXAMPLE 1 In this run for this example, the feed mixture was passedthrough a bed of an alpha-alumina supported iridium catalyst containing1.5 weight percent iridium. The results of this run are summarized inTable I below.

EXAMPLE 2 In the run for this example, the feed mixture was passedthrough a bed of a silica supported iridium catalyst containing fiveweight percent iridium. The silica support was a commercial Cab-O-Silmaterial obtained from the Cabot Company. The catalyst was prepared by 3the method of incipient wetness by contacting the Cab- O-Sil with anaqueous solution of an appropriate amount of an iridium salt, i.e. HIrCl drying the material and reducing at 300 C. in hydrogen to convertthe salt to metallic iridium. The results of this run are summarized inTable I below.

Referring to Table I below, the percent selectivity to the usefuloxygenated products Was about the same (30.5 v. 28) for thealpha-alumina supported iridium catalyst (Example 1) as with the silicasupported iridium catalyst (Example 2) at about the same oxidation rate.

A series of runs was made by passing a feed mixture containing varyingamounts of oxygen, propylene and helium over the catalyst of Example 1at varying conditions to determine the effect of changes in temperatureand feed composition on reaction rate and selectivity to the productionof acetic acid, acetone and acrolein. The results of this series of runsare given in Table II below.

Referring to Table II below, it can be seen that the selectivity to theproduction of acetic acid, acetone and acrolein decreases slightly ingoing from 100 to 123 C.

propylene with a gas containing free molecular oxygen in the contactpresence of iridium metal.

2. A process according to claim 1 wherein the reaction occurs at atemperature from 50 to 300 C., a partial pressure of oxygen above about0.5 pounds .per square inch absolute and a propylene space velocity fromabout one to about 2000.

3. The process of claim 2 wherein the temperature in the reaction zoneis maintained in the range of about 80 to about 200 C.

4. A process according to claim 2 wherein the partial pressure of oxygenis within the range of about 0.5 to about 10 pounds per square inchabsolute.

5. The process of claim 1 wherein the iridium metal is deposited on asupport.

6. A process according to claim 5 wherein the reaction occurs at atemperature from 50 to 300 C. and the amount of iridium metal is from0.2 to 30 weight percent of the total catalyst.

TABLE I.-PRODUCTS OF PROPYLENE OXIDATION OVER SUPPORTED IRIDIUM AT ATOTAL PRESSURE OF ABOUT 740 Percent selectivity I to Total percentOxidation 1 Acrylic selectivity Oxygen Propylene Rate us Acrolcin touseful Wt. percent pressure pressure Temp. Volumes, Acetic proplonicplus oxygenated Example No. Metal Support metal (mm) (mm.) C.)C3H6/1'HII1. acid acid acetone products 1 Ir a-Alzog 1. 5 131 7 135 0.15 0. 5 6. 0 30. 5 2 Ir SiO 5.0 61 21 135 0.18 21 3 4 28 1 The oxidationrate is defined as the rate at which propylene is converted to allproducts in volumes (STP) per minute, e.g. if the propylene is passedover the catalyst at two volumes per minute and it is found that 1.8volumes per minute is recovered unchanged, then the oxidation rate is2.0

minus 1.8, or 0.2 volumes per minute.

a The prrcent selectivity is defined as the percent of the propyleneoxidized which is converted to the given product.

TABLE II.DEPENDENDENCE OF RATE AND SELECTIVI'IY OF PRO- PYLENE OXIDATIONOVER 1.5% Ir/a-AlzOa 0N PROCESS VARIABLES Percent selectivity to- OxygenPropylene Oxidation Ex. Temp. pressure pressure rate. Acetic No. 0.)(mm.) (mm.) vols/min. acid Acetone Acrolein A See Table I footnotes.

(compare Examples 3-8). The rate, however, increases considerably withthe increase in temperature (again compare Examples 3-8). The rate ofoxidation increases with propylene pressure as can be seen by comparingEX- amples 9-13. The oxidation rate is not greatly affected by theoxygen partial pressure but the percent selectivity to useful oxygenatedproducts increases with an increase in oxygen partial pressure as can beseen by a comparison of Runs 14-18.

Obviously, many modifications and variations of the invention ashereinabove can be made Without departing from the spirit and the scopethereof, and such modifications and variations are intended to beincluded within the scope of this invention.

We claim:

1. A process for the production of a product comprising acetic acid,which process comprises reacting 7. A process according to claim 6wherein the support is alpha-alumina.

8. A process according to claim 6 wherein the support LORRAINE A.WEINBERGER, Primary Examiner R. D. KELLY, Assistant Examiner U.S. Cl.X.R.

